The invention relates to a method for measuring geometric characteristics of workpieces on coordinate measuring machines or digitalizing machines by means of measuring apparatus, which supply measurement information as to geometric characteristics of the workpiece relative to the measuring apparatus, which is, in short, often characterized as measuring sensors.
Initial Comment
In the following, the abbreviation CMA is used for coordinate measuring machines as well as for digital machines.
Today, mostly CMAs are used for detecting three-dimensional geometric variables. These CMAs are mostly equipped with switching probes which, when contacting the surface, generate an electric signal. With this signal, the positions of the individual linear or rotational axes of the CMAs are frozen, that is, the axial positions are copied into an output register. This data is then used in the known kinematic of the CMA, including the probe, to generate a measuring point in a stationary coordinate system. When generating a measuring point, partially more extensive error compensations are carried out, such as probe ball radius correction, geometric or thermal machine position error corrections. The basis for the fault correction and, in part, also for the determination of the kinematics, is mostly a calibration process.
If several points are sequentially measured in the same fixed coordinate system, the relative position of the measuring points in the spatially fixed coordinate system to each other is a measure for the relative position of the contact points on the workpiece surface. In this way, geometric quantities can be determined on the workpiece.
To an increasing extent, measuring sensors are preferably used in lieu of switching probes for detecting free-form surfaces. Contacting as well as contactless sensors are used. These sensors determine the position of the measuring point on the workpiece relative to the recording thereof, that is, in a sensor fixed coordinate system, hereinafter known as RST coordinate system. A sequence of measuring points is, as a rule, detected during the movement of the CMA and one speaks of a xe2x80x9cscanningxe2x80x9d operation.
For computing a measurement point in a stationary coordinate system, the sensor information in the RST coordinate system must be coupled in time synchronism with the information for the position of the sensor in the stationary coordinate system. In methods known at the present time, this takes place in that the result of the measurement of the sensor in the RST coordinate system is detected in time as close as possible to the result of the sensor position determination by the CMA in the stationary coordinate system in a mostly digital signal processing unit and, thereafter, a measuring point in the stationary coordinate system is computed from these two individual informations.
In the known method, so that the information from the sensor and the CMA originates indeed at the same time point, the times for obtaining the measurement value, the measurement value processing and the measurement value transmission, must have the same magnitude for the sensor and the CMA so that the time synchronous coupling of the results corresponds also to the same measurement time point.
Otherwise, the information of the sensor would be coupled to a position of the sensor which it did not have at the time point of obtaining the information and this would lead to an incorrect computation of the measuring points with the exception of some special cases.
There are systems known wherein, for unequal but respectively constant signal running times, a correction is preferably carried out via a delay path of the more rapid information utilizing a ring buffer; these systems, however, are subject to disturbances because the assumption of constant signal running times (especially for optical sensors having complex signal processing) cannot be guaranteed and the signal transmission time up to the point of the signal processing unit (at which both informations are detected simultaneously) is generally not constant. This applies primarily when the signal processing unit used is a complex multitasking operating system such as Windows NT.
Furthermore, all systems have as a precondition that the measurement data rate of sensor and CMA is the same, that is, a result from the sensor is coupled to a result from the CMA.
The method of the invention overcomes these disadvantages in that the time point of the measurement is determined by an electrical signal (trigger) for the CMA as well as for the sensor.
All CMAs, which are designed for operation with a switching probe, permit this function in that the trigger signal is treated like a signal of a switching probe. The sensors usable in accordance with the invention likewise have to exhibit a possibility for synchronization. This can be a trigger input for the exact determination of the measurement time point as well as an output signal (SYNC), which indicates the exact measurement time point. In the first case, a measurement data detection system needs to contain a trigger source in accordance with the method of the invention; in the second case, the sensor itself would function as a trigger source.
A signal processing unit in accordance with the method of the invention ensures that precisely one measurement result from the sensor and one measurement result from the CMA per trigger is further processed.
In contrast to known methods, different signal running times no longer have an effect because, in the signal processing unit, the simultaneousness of the measurement is determined by the sequence of the results and no longer by the time point of the detection of the results. Stated otherwise, the n-th result after an initialization of the CMA and the n-th result after an initialization of the sensor originate reliably at the same time point because they were triggered by the same trigger.
Since especially contactless sensors are essentially more dynamic than the CMA, different measurement data rates of sensor and CMA are purposeful and possible with the method according to the invention, preferably when the ratio of the measurement data rates is a whole number. In this case, for the computation of measurement points, the results of the CMA are interpolated in accordance with known methods so that, after the interpolation, the same amount of information is present from the CMA and from the sensor and can be processed further as with the same measurement data rate.
In order to make the identification more reliable as to which result belongs to the n-th trigger (for example, to detect possibly occurring signal transmission defects), the sensors as well as the CMA should, in a preferred variation of the method of the invention, in addition to its other informations, transmit a TAN-NR (transaction identifier) which is increased with each trigger and is again reset when a maximum number is exceeded. In this way, for a coupling of the CMA data and the sensor data, synchronism can be checked in a simple manner and synchronization reached.
In a preferred variation, at least the sensor and preferably also the CMA have a FIFO output memory which ensures that even under not real-time capable operating systems (such as Windows NT), a complete and clear transmission of each individual information per trigger is ensured. Especially with the high data rate, which is possible for contactless sensors, it is thereby prevented that the signal processing unit (because of loading with other tasks) cannot process the signal transmission within the time pregiven by the measurement data rate and therefore individual informations are not even processed further.
The FIFO output memory, especially in combination with the TAN-NR, permits, on the one hand, a very disturbance-free operation compared to present-day solutions and, on the other hand, it permits the information from sensors or CMA to be transmitted in selectable block sizes for several triggers instead of individually per trigger which significantly reduces the load of the signal processing unit because of the data transmission.
The method according to the invention therefore permits a significant increase of the measurement data rate with a simultaneous increase with respect to disturbance reliability and a reduction of the load of the signal processing unit without a significant increase of complexity of present-day conventional CMAs.